1. Technical Field
The present disclosure relates to a common mode noise cancellation circuit, in particular, to the common mode noise cancellation circuit for unbalanced signals.
2. Description of Related Art
Interference caused by common mode noise is a common issue in an electronic system. Such, phenomenon occurs when a positive/negative terminal and grounding of a signal are interfered with the same noise. Interference caused by the common mode noise is likely to have a negative impact on a signal and a system, such as downgrading signal quality and system performance. A conventional solution to common mode noise interference is to utilize fully differential signals, so as to eliminate the noise by subtracting the positive terminal with the negative terminal of the interfered signal. However, if interference caused by the common mode noise occurs in an unbalanced signal system, meaning the signal and the grounding are both interfered simultaneously. In this case, interferences on both terminals are difficult to be removed.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional example of amplifier system with unbalanced wires connection. A first signal terminal S1 and a second signal terminal G1 of an amplifier A of a signal source 1 represent an output signal and grounding of the amplifier A respectively. Signals of the first signal terminal S1 and the second signal terminal G1 can be interfered by external common mode noise Nc. The signals are transmitted to a receiving circuit 2 via wires L1 and L2. The RL1 and RL2 represent the line resistors of L1 and L2 respectively. The receiving circuit 2 comprises an amplifier B. Assuming potential of a grounding terminal GND2 of the receiving circuit 2 is an ideal grounding potential, hence a signal of a node G2 of the amplifier B is clean and possesses zero potential. Assuming the input impedance of the amplifier B is infinite, the signal at the node S2 will almost be identical to a signal of the first signal terminal S1, and will still carry the common mode noise. After amplification of the amplifier B, common mode noise at node S3 is also amplified on load 3.
Please refer to FIG. 2. FIG. 2 is a diagram illustrating the waveform of amplifier system with unbalanced wires connection shown in FIG. 1. Signals at the first signal terminal S1 and the second signal terminal G1 are a pair of unbalanced signals, which are coupled with the same noise simultaneously, but a differential signals (S1−G1) of the first signal terminal S1 and the second signal terminal G1 can eliminate such common mode noise. Since the signal of the node G2 is clean ground, so when the signal is transmitted to the amplifier B, the resistor RL2 separates signals of the second signal terminal G1 from signals of the node G2 and maintains a certain potential difference, and it can be assumed that grounding to which the second signal terminal G1 is coupled possesses an ideal zero potential. Assuming an input of the amplifier B is very high impedance, noise at the node S2 is not attenuated. Hence it can be observed that an original signal added common mode noise are both amplified and feeds to the load 3 at node S3. The signal to noise ratio (SNR) of such system will get worse.
According to above mentioned issues, if the frequency of the common mode noise is high, a common mode choke can be utilized to eliminate or attenuate the common mode noise. However, if the noise frequency is low, the common mode choke is almost invalid to reduce the noise since impedance of an inductive component such as the common mode choke is very low in low frequency. For example if the common mode noise is within the audio band, it is very difficult to remove the noise with common mode choke.
Further, since common mode noise for unbalanced signals is not an easy issue to be solved, a conventional solution is to convert the unbalanced signal to a balanced or said fully differential signals during signal processing. Such process, however, can be complex and costly. Hence interference caused by common mode noise, especially in low frequency, is a problem which is difficult to be solved. For instance, in GSM (Global System for Mobile Communications), interference in 217 Hz (Hertz) caused by TDD (Time Division Duplex) is a common issue. Since the frequency of such interference is within audio band, voice signal of a mobile phone is directly interfered. A general problem is that when using a wired earphone on GSM mobile phone, TDD noise possibly couples to the earphone via wireless radio radiation or wired power noise coupling from the handset. The TDD noise direct couples to both signal and ground lines of the earphone, it is a kind of common mode noise interference. As a result, it is important for the driving circuitry and the circuit layout between the mobile phone and the earphone to cope/tolerate with the common mode noise interference. Earphone wiring can only utilize metallic shielding to block out as much common mode noise interference caused by radiation as possible, or even utilize a notch filter to eliminate the interference signal of such specific frequency. A device such as the common mode choke is almost invalid to common mode interference since the noise is within the low frequency audio band. Therefore, it is difficult and costly to eliminate the interference caused by common mode noise, especially for low frequency signals.
Another example is a conventional electronic product using commercial power. Such product is also prone to power source interferences (such as common mode noise interference in 60 Hz from a power system) if designed badly or is with poor grounding. Such interference is likely to affect other peripherals connecting to the electronic product. The peripherals can be easily affected unless the peripherals are designed for anti-interference.